fix(ecs): Allow scheduling DAEMON services even if no EC2 capacity attached to cluster (#25306) (#25328)

It can be useful to allow DAEMON services to be scheduled to a cluster even if there is no EC2 capacity attached to it yet, under the assumption that capacity will be attached to the cluster later. We no longer fail validation if this happens.

Fixes #25306

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: otterley

Diff for: packages/aws-cdk-lib/aws-ecs/lib/ec2/ec2-service.ts

@@ -249,7 +249,7 @@ export class Ec2Service extends BaseService implements IEc2Service {
    */
   private validateEc2Service(): string[] {
     const ret = new Array<string>();
-    if (!this.cluster.hasEc2Capacity) {
+    if (!this.daemon && !this.cluster.hasEc2Capacity) {
       ret.push('Cluster for this service needs Ec2 capacity. Call addXxxCapacity() on the cluster.');
     }
     return ret;

Diff for: packages/aws-cdk-lib/aws-ecs/test/ec2/ec2-service.test.ts

@@ -2093,8 +2093,51 @@ describe('ec2 service', () => {
       expect(() => {
         service.addPlacementStrategies(PlacementStrategy.packedBy(ecs.BinPackResource.MEMORY));
       }).toThrow();
+    });
+
+    test('throws an exception if non-DAEMON service is added but no EC2 capacity is associated with the cluster', () => {
+      // GIVEN
+      const stack = new cdk.Stack();
+      const vpc = new ec2.Vpc(stack, 'MyVpc', {});
+      const cluster = new ecs.Cluster(stack, 'EcsCluster', { vpc });
+      const taskDefinition = new ecs.Ec2TaskDefinition(stack, 'Ec2TaskDef');
+
+      taskDefinition.addContainer('web', {
+        image: ecs.ContainerImage.fromRegistry('amazon/amazon-ecs-sample'),
+        memoryLimitMiB: 512,
+      });
+
+      new ecs.Ec2Service(stack, 'Ec2Service', {
+        cluster,
+        taskDefinition,
+      });
 
+      expect(() => {
+        Template.fromStack(stack);
+      }).toThrow(/Cluster for this service needs Ec2 capacity/);
+    });
 
+    test('does not throw an exception if DAEMON service is added but no EC2 capacity is associated with the cluster', () => {
+      // GIVEN
+      const stack = new cdk.Stack();
+      const vpc = new ec2.Vpc(stack, 'MyVpc', {});
+      const cluster = new ecs.Cluster(stack, 'EcsCluster', { vpc });
+      const taskDefinition = new ecs.Ec2TaskDefinition(stack, 'Ec2TaskDef');
+
+      taskDefinition.addContainer('web', {
+        image: ecs.ContainerImage.fromRegistry('amazon/amazon-ecs-sample'),
+        memoryLimitMiB: 512,
+      });
+
+      new ecs.Ec2Service(stack, 'Ec2Service', {
+        cluster,
+        taskDefinition,
+        daemon: true,
+      });
+
+      expect(() => {
+        Template.fromStack(stack);
+      }).not.toThrow();
     });
   });